1. Field of the Invention
The present invention relates to a die cushion apparatus which drives to raise or lower a cushion pad while applying an upward energizing force, and more particularly to a die cushion apparatus which alleviates an impact generated in a press machine.
2. Description of the Related Art
A die cushion apparatus (hereinafter merely referred to as a “die cushion”) for crease pressing in a drawing work is provided in a press machine. A conventional die cushion generates a cushion pressure while driving to raise or lower a cushion pad by using a hydraulic pressure or an air pressure. In order to prevent a work from being damaged or strained by raising drawing workability of the die cushion, it is necessary to control the cushion pressure of the die cushion in high accuracy, and particularly, it is necessary to control the cushion pressure at the time of lowering operation of the cushion pad in high accuracy.
The die cushion using only the air pressure cannot control the cushion pressure in high accuracy at the time of operating the cushion pad. The die cushion using the hydraulic pressure can control the cushion pressure in high accuracy at the time of operating the cushion pad under the control of a hydraulic pressure. However, a structure of a hydraulic apparatus is complicated, and there is a difficult point of needing severe maintenance and management. Therefore, recently, a die cushion having an electric servomotor which has a simple structure and which does not need severe maintenance and management is noted.
When a slide lowers so that an upper die is brought into contact with a work in the case of press working, there arises a state that the slide of large weight collides with the cushion pad. Since a large load is operated instantaneously at the cushion pad, a large impact occurs in the press machine.
FIG. 11 is a view showing the relationship between a load operated at the cushion pad and a time in a conventional apparatus. Here, a load change after the upper die is contacted with the work is shown. According to FIG. 11, it is understood that, when the upper die is contacted with the work, an overshoot further exceeding a normal value is generated.
In order to alleviate such an impact, the cushion pad is lowered synchronously with the lowering operation of the slide to lower the cushion pad so that a relative speed of the upper die side (slide) to the work side (cushion pad) is decelerated in a control aspect, so called a preliminary acceleration is performed.
If a large impact continuously arises, there occurs a problem that a wearing speed of a mold is accelerated to cause a cost to rise. If the impact is large, an impact sound generated in this case increases, and a problem of a noise also occurs. In order to avoid these problems, it is desired to reduce the impact as much as possible. Therefore, a devise is desired not only in a field of the control but also in a structure of the die cushion.
The present invention is made in view of such a fact, and provides a die cushion having a structure which can alleviate an impact generated in a press machine.